Electrical Troubleshooting Quiz

In the last 18 months, a 75kVA dry-type transformer has had to be replaced twice. It's mounted in the ceiling of the facility's data processing room. There is a small UPS system in the room, but it doesn't provide enough ride-through for the time it takes to complete a transformer replacement.

A proposed solution was to run an alternative power line to the room, but replacing the transformer would still be highly disruptive. It's overhead work, so the people who normally work in that room would need to move their workstations and some equipment out to the corners.

How can you prevent a repeat of this, with the transformer that's now installed?

You need to move that transformer so it's not over the work areas. Transformers are often tucked into the ceiling to save room, but thermal considerations can render that a poor solution. Wherever you mount the new transformer, ensure it has adequate airflow around it. Install temperature monitoring, and for the first year conduct monthly thermal imaging.

Because the room is full of switching power supplies, use this project as an opportunity to derate the transformer and the conductors for harmonic content. If the derating calculations show the 75kVA is undersized, you've found your root problem. Look at the transformer secondaries with power analyzer and discuss your findings with the UPS vendor.

The alternative line is a good idea. But only if you install an automatic transfer switch to ensure smooth transition of the load to this secondary source and back again.

All good solutions. I might use a K factor transformer to replace the damaged unit. They're a bit heavier, especially the neutral core. Maybe up size the neutral conductor(s) if they are not full sized (same as the phase conductors) You might also consider a software up grade to your UPS system if the numbers work to service the connected loads. I've gotten out of some tight spots with this one just by having the UPS vendor tweek their software to give me a bit more head room on the system.

We have a similar problem with 30kVA UPS unit fed from 30kVA transformer. For the past year we had to replace the transformer twice. Still in process to figure out what cause the problem. The first taught was the harmonics, generated by the computer loads, since the UPS is built to mitigate the harmonics. We just found that we have UPS which is not working properly, it is possible to damage the transformer. Did you try to meseaure the harmonics and the load on the secondary of the 75 kVA transformer and what results did you get? Thank you.

Any upstream device serving a UPS needs to be sized at least 1.5 times the size of the UPS. When the UPS becomes the source for the load and is recharging the batteries at the same time, the one-to-one rule doesn't work. The 30KVA transformer is too small.

Guys what is the reason for failure?. Evaluate the transformer to find out if it is oveheating or shorted windings . Could simply be no air circulation.
A UPS will not damage a transformer, improperly installed or undersized transfomrer will fail. Although type K would eliminate the Harmonics concern it does not gurantee failure free operation.

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